Magneto-optical recording medium and method for producing the same

Abstract

A magneto-optical recording medium having a recording layer made of a bismuth-substituted garnet and deposited by a sputtering over an amorphous substrate of aluminosilicate or borosilicate glass, the recording medium including a garnet underlayer of either a double-layer or single-layer structure. The formation of an underlayer of a double-layer structure made of garnet reduces the level of noise due to light scattering at crystalline boundaries in the garnet film. The formation of an underlayer of a single-layer structure made of a garnet having a lower crystallization temperature than a YIG film is also effective to noise-level reduction. An underlayer has a surface smoothness corresponding to a roughness of approximately 10 Å, which is improved from the surface smoothness of a conventional underlayer comprising a YIG film.

We claim: 
     
       1. A magneto-optical recording medium comprising: an amorphous substrate which is transparent to laser light;   a first layer formed on a surface of the substrate, the first layer being amorphous and having a chemical composition corresponding to a first garnet;   a second layer formed on the first layer, the second layer being composed of crystals of a second garnet having a crystallization temperature lower than the softening point of the substrate and an average surface roughness of less than 40 Å; and   a third layer formed on the second layer, the third layer being composed of a bismuth garnet having it axis of easy magnetization perpendicular to the surface of the substrate.   
     
     
       2. A magneto-optical recording medium according to claim 1, wherein the second garnet has a crystallization temperature lower than the crystallization temperature of the first garnet. 
     
     
       3. A magneto-optical recording medium according to claim 2, wherein the second garnet has the formula A x  Y 3-x  Fe 5  O 12 , where 0≦x≦3 and A is a rare earth element. 
     
     
       4. A magneto-optical recording medium according to claim 3, wherein the first garnet in the first layer has the formula A x  Y 3-x  Ga 5  O 12 , where 0≦x≦3 and A is a rare earth element. 
     
     
       5. A magneto-optical recording medium comprising: an amorphous substrate which is transparent to laser light;   a first layer formed on a surface of the substrate, the first layer being composed of a magnetic garnet having a crystallization temperature lower than the softening point of the substrate;   a second layer formed on the first layer, the second layer being composed of a non-magnetic garnet having a crystallization temperature lower than the softening point of the substrate and an average surface roughness of less than 40 Å; and   a third layer formed on the second layer, the third layer being composed of a bismuth garnet having it axis of easy magnetization perpendicular to the surface of the substrate.   
     
     
       6. A magneto-optical recording medium according to claim 5, wherein the garnet forming the second layer has the formula A x  Y 3-x  Ga z  Fe 5-z  O 12 , where A indicates a rare earth element, 0≦x≦3 and 1.5≦z<s5. 
     
     
       7. A magneto-optical recording medium according to claim 5, wherein the non-magnetic garnet is a bismuth garnet of a formula Bi x  R 3-x  Ga z  Fe 5-z  O 12 , where 0<x<3, 1.5≦z<5 and R is a rare earth element. 
     
     
       8. A magneto-optical recording medium according to claim 5, wherein the garnet forming the first layer is composed of an aluminum garnet. 
     
     
       9. A magneto-optical recording medium according to claim 1 or 5, wherein the bismuth garnet in the third layer has the formula Bi x  R 3-x  Ga z  Fe 5-z  O 12 , where 0<x<3, 0≦z<1 and R is a rare earth element. 
     
     
       10. A magneto-optical recording medium according to claim 1 or 5, wherein the amorphous substrate is an aluminosilicate glass or a borosilicate glass. 
     
     
       11. A magneto-optical recording medium comprising: an amorphous substrate which is transparent to laser light;   a first layer formed on a surface of the substrate, the first layer being composed of a magnetic garnet having a crystallization temperature lower than the softening point of the substrate and its axis of easy magnetization in parallel to the surface of the substrate;   a second layer formed on the first layer, the second layer being composed of a non-magnetic garnet having a crystallization temperature lower than the softening point of the substrate and an average surface roughness less than 40 Å; and   a third layer formed on the second layer, the third layer being composed of a bismuth garnet and having its axis of easy magnetization perpendicular to the surface of the substrate.